do not call (av_)abort()

Originally committed as revision 3544 to svn://svn.ffmpeg.org/ffmpeg/trunk
diff --git a/libavformat/mpeg.c b/libavformat/mpeg.c
index adf871a..cf6cb0b 100644
--- a/libavformat/mpeg.c
+++ b/libavformat/mpeg.c
@@ -314,7 +314,7 @@
             s->video_bound++;
             break;
         default:
-            av_abort();
+            return -1;
         }
     }
     /* if no SCR, use first stream (audio) */